Some consider Iago to be not only Shakespeare's best villain but also one of literature's most effective ones. Iago deserves this distinction for several reasons.

He is extremely skilled in the art of subterfuge. Iago announces to Roderigo and the audience in Act I, Scene 1,

"I will wear my heart upon my sleeve/ For daws to peck at. I am not what I am" (66-67).

Iago knows how to control his hatred of Othello and Cassio so that he can convince them both that he has their best interest in mind. In fact, his ability to appear loyal to everyone he talks to is his most effective attribute.

Iago also seems to be amoral, so much so that some have argued that if he were a real person, he would most likely be classified as a psychopath. He demonstrates no remorse for his evil-doing and seems to relish being a devilish character. Even at the play's end, when the tragedy he produced is discovered, he will offer no explanation. He tells Othello,

"Demand me nothing. What you know, you know. / From this time forth I never will speak word" (5.2.316-317).

Not only does he not explain to Othello why he manipulated him, but he also provides no real explanation to the audience. Admittedly, he references a rumor that Othello slept with Emilia and that he was passed over for the military position that he wants, but he has no basis for either claim--Cassio most likely would have gotten the position whether Iago was around or not.

Finally, Iago is actually a successful villian. He is not dead at the play's ending. He has accomplished much of what he wanted to do (ruining Othello and his relationship with Desdemona and manipulating Cassio into hurting his own reputation). Many epic battles between good and evil feature the villain getting his just desserts in the end, but not so with Iago. The audience has to trust that those who did not stop Iago's original plan (Montano, Cassio, Lodovico, etc.) will be able to bring him to justice without his weaseling his way out of punishment.